Statement
The standing crop biomass (expressed as the total dry weight of organisms present at anyone time) which can be supported by a steady flow of energy in a food chain depends to a considerable extent on the size of the individual organisms. The smaller the organisms the greater its metabolism per gram of biomass. Con- sequently, the smaller the organism, the smaller the biomass which can be supported at a particular trophic level in the ecosystem.
Conversely, the larger the organism, the larger the standing crop biomass. Thus, the amount of bacteria present at anyone time would be very much smaller than the "crop" of fish or mammals even though the energy utilization was the same for both groups.
Explanation and examples
The metabolism per gram of biomass of the small plants and animals such as algae, bacteria, and protozoa is immensely greater than the metabolic rate of large organisms such as trees and verte- brates. This applies to both photosynthesis and respiration. In many cases the important parts of the community metabolically are not the few great conspicuous organisms but the numerous tiny organisms which are often invisible to the naked eye. Thus, the tiny algae (phytoplankton), comprising only a few pounds per acre at anyone moment in a lake, can have as great a metabo-
ENERGY IN ECOLOGICAL SYSTEMS: §3 57
lism as a much larger volume of trees in a forest or hay in a meadow. Likewise, a few pounds of small crustacea (zooplank- ton) "grazing" on the algae can have a total respiration equal to many pounds of cows in a pasture.
The rate of metabolism of organisms or association of organisms is often estimated by measuring the rate at which oxygen is con- sumed (or produced in the case of photosynthesis). There is a broad general tendency for the metabolic rate per organism in animals to increase as the two-thirds power of the volume (or weight) increases, or the metabolic rate per gram biomass to de- crease inversely as the length (Zeuthen, 1953). A similar re- lationship appears to exist in plants, although structural differ- ences in plants and animals (see below) make direct comparisons in terms of volume and length difficult. These relations are shown in Figure 13 by the smoothed Hnes which indicate in an approxi- mate manner the relation between size and metabolism. Various theories proposed to accolmt for these trends have centered around diffusion processes; larger organisms have less surface area per gram through which diffusion processes might occur.
However, the real explanation for the relationship between size and metabolism has not been agreed upon. Comparisons, of course, should be made at similar temperatures because meta- bolic rates are usually greater at higher temperatures than at lower temperatures (except with temperature adaptation; see page 90).
It should be pointed out that when organisms of the same general order of magnitude in size are compared the linear rela- tionship shown in Figure 13 does not always hold. This is to be expected since there are many factors, secondary to size, which affect the rate of metabolism. For example, it is well known that warmblooded vertebrates have a greater respiration rate than cold- blooded vertebrates of the same size. However, the difference is actually relatively small as compared with the difference between a vertebrate and a bacterium. Thus, given the same amount of available food energy, the standing crop of coldblooded herbivor- Ous fish in a pond may be of the same order of magnitude as that of warmblooded herbivorous mammals on land. In the study of size-metabolism in plants, it is often difficult to decide what con- stitutes an "individual." Thus, we may commonly regard a large tree as one individual, but actually the leaves may act as "fWlc- tional individuals" as far as size-surface area relationships are concerned. In a recent study of various species of seaweeds (large
58 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CR. 3
... TULIIIAl..lv AOAPTID 'HOTO .TNT . . '"
.1
.01
~M/ /HR
.001 OXYGEN
.0001
10"
10-T::-:-_---::"::-_--< _____ ---+ _ _ _ - _ . . . J
.0001 .001 .01 .1 I 10 100
THICKNESS OF LEAF OR ORGANISM IN CM
CLAOOPHORA V
02 z
r:: u
:::>
o o
a:: ~ I CHOaO~US SARGASSUM.
(f)
~ . FUCUS
~ . CHO"O.
20 50 100 200 500
SURFACE/VOLUME RATIO CM2/CM3
Figure J 3. Metabolism per gram biomass as a function of orgauismlll size.
From top to bottom the curves represent: photosynthetic rate per gram biomass in a variety of algae and leafy plants in relation to length or thick- ness of leaf, respiration per gram biomass in heterotrophic organisms (animals and bacteria) in relation to the length of the organism, and gross production of several species of large marine seaweeds in reJation to smfuce-to-volume ratio. In each case small organisms or thin organisms with a high surface-to- volume ratio have higher rates of metabolism per gram than large or thick organisms. Upper graph after H. T. Odum (U)56a) from data of Verduin and Zeuthell. Lower graph after E. P. Odum et a1. (1958).
ENERGY IN ECOLOGICAL SYSTEMS: §4 59 multicellular algae), we found (see Figure 13) that species with thin or narrow "branches" (and consequently a high surface-to- volume ratio) had a higher rate per gram biomass of food manu- facture, respiration, and uptake of radioactive phosphorus from the water than did species with thick branches (E. P. Odum.
Kuenzler and Blunt, 1958). Thus, in this case the "branches" 01
even the individual cells were "functional individuals" and not the whole "plant," which might include numerous "branches"
attached to the substrate by a Single holdfast.
The inverse relationship between size and metabolism may also be observed in the ontogeny of a single species. Eggs, for example, usually show a higher rate per gram than the larger adults. In data reported by Hunter and Vernberg (1955), the metabolism per gram of trematode parasites was found to be ten times less than that of the smaIllm'val cercariae.
4. Trophic structure and ecological pyramids
Statement
The interaction of the food chain phenomena (energy loss at C'aeh transfer) and the size-metaholism relationship results in com- munities having a definite trophic' ~tru.ctU1'e. which is often charac- teristic of a particular type of ecosystem (lake, forest, coral reef, pasture, etc.). Trophic structure may be measured and described either in terms of the standing crop per unit area or in terms of the energy fixed per unit area per unit. ti-q}e at successive trophic levels.
Trophic structure and also trophic function may be shown graphi- cally hy means of ecological pyramids in which the first or pro- dllcer level forms the base and successive levels the tiers which make up the apex. Ecological pyramids may be of three general types: (1) the pyramid of numbets in which the number of indi- vidual organisms is depicted, (2) the pyramid of biomass based on the total dry weight, caloric value, or other measure of the total amolmt of living material and (3) the pyramid
of
energy in which the rate of energy How and/or "productivity" at successive trophiC levels is shown. The numbers and biomass pyramids may be inverted (or partly so), that is, the base may be smaller than one or more of the upper tiers if producer organisms average smaller than consumers in individual size. On the other hand, the energy pyramid must always take a true upright pyramid shape, provided all sources of food energy in the system are considered.60 BASIC ECOLOCICAL PRINCIPLES AND CONCEPTS: CR. 3
Explanation
The pyramid of numbers is actually the result of three phenom- ena which usually operate simultaneously. One of these phenom- ena is the familiar geometrical fact that a great many small units are required to equal the mass of one big unit, regardless of whether the units are organisms or building blocks. Thus, even if the weight of large organisms was equal to the weight of the smaller ones, the number of smaller organisms would be vastly greater than that of the larger ones. Because of the geometry, therefore, the existence of a valid pyramid of numbers in a natural group of organisms does not necessarily mean that there is less of the larger organisms on a weight basis.
The second phenomenon contributing to the pattern of many small organisms and few large ones is the food chain. As painted out in Section 2, useful energy is always lost (into the form of heat) in the transfer through each step in the food chain. Con- sequently, except where there are imports or exports of organic matter, there is much less energy available to the higher trophic levels. The third factor involved in the pyramid of numbers is the inverse size-metabolic rate pattern discussed in the previous section.
Actually, the pyramid of numbers is not very fundamental or instructive as an illustrative device since the relative effects of the
"geometric," "food chain" and "size" factors are not indicated.
The form of the numbers pyramid will vary widely with different communities, depending on whether producing individuals are smal1 (phytoplankton, grass) or large (oak trees). Likewise, num- bers vary so widely that it is difficult to show the whole community on the same numerical scale. This does not mean that the number of individuals present is of no interest, but rather that such data are probably best presented in tabular form.
The pyramid of biomass is of more fundamental interest since the "geometric" factor is eliminated, and the quantitative relations of the "standing crop" are well shown. In general, the biomass pyramid gives a rough picture of the overall effect of the food chain relationships for the ecological group as a whole. When the total weight of individuals at successive trophic levels is plotted, a gradually sloping pyramid may be expected as long as the size of the organisms does not differ greatly. However, if organisms of lower levels average much smaller than those of higher levels, the biomass pyramid may be inverted. For example, where size of the producers is very small and that of the consumers large, the total
ENERGY IN ECOLOGICAL SYSTEMS: §4 61 weight of the latter may be greater at anyone moment. In such cases, even though more energy is being passed through the pro- ducer trophic level than through consumer levels (which must always be the case), the rapid metabolism and tumover of the small producer organisms accomplish a larger output with a smaller standing crop biomass.
From the foregoing discussion it becomes clear that the number and weight of organisms which can be supported at any level in any situation depends, not on the amount of fixed energy present at anyone time in the level just below, but rather on the rate at which food is being produced. The important time factor is intro- duced in the pyramid of energy which depicts the amount of organisms (usually expressed in terms of calories, a widely used unit of energy) produced within a given unit of time. Of the three types of ecological pyramids, the energy pyramid gives by far the best overall picture of the functional nature of communities; it actually shows "what goes on" in the living part of the ecosystem.
Tn contrast with the numbers and biomass pyramids which are pictures of the standing states, i.e., organisms present at anyone moment, the energy pyramid is a picture of the rates of passage of food mass through the food chain. Its shape is not affected by variations in the size and metabolic rate of individuals, and, if all sources of energy are considered, it must always be "right side up"
because of thc second law of thermodynamics. As will be dis- cussed in more detail in the next section, the size of the various tiers is a direct measure of energy flow at successive levels. Theo- retically, tlle fmm of the energy pyramid should be consistent and characteristic of a particular ecosystem if a reasonable time inter- val is considered. The one drawback to this form of graphic sum- mary is that the data for its construction are often difficult to obtain.
Examples
The several types of ecological pyramids which we have dis- cussed are illustrated in Figures 14 to 17. These are all constructed from published data, as indicated. In Figure 14, the numbers of all macroscopic organisms in one acre of a grassland community are arranged according to broad trophic levels to demonstrate an
"upright" type of pyramid of numbers in which producers (plants) are individually more numerous than consumers (herbivorous in- sects, etc.) which in tum are associated with a very few tertiary consumers (birds and moles). As pointed out in the above state-
62 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CR. 3
PRODUCERS 5,842,424
Figure 14. An example of the "pyramid of num bets." The number of organisms (exclusive of decomposers) in a bluegrass field are arranged accord·
ing to trophic levels as follows: Produccrs - green plants. C-1, herbivorous invertebrates. C-2, spiders, ants, predatory beetles, etc. C-3 birds and moles.
(Plant data, Evans and Cain, 1952; animal data, Wolcott, 1937.)
ment, the relationship between the numher of producers and pri- mary consumers would be completely l'fwersed in a forest where there would obviouslv be more insects than trees. The number of microorganism (deco~posers) in grassland was not determined in the studies on which Figure J 3 is based; their number, of course, would be extremely large and require a different scale to show on the same diagram with the other components.
Figure 15 illustrates biomass pyramids for a wide variety of communities in marine, freshwater and terrestrial envirOl1ment~.
The diagram for the rich and beautiful Silver Springs, Flolida, which is visited by thousands of tourists annually, is especially interesting since an estimate of all of the community, including the decomposers, is shown. Beds of freshwater eelgrass (Sag-it- tal'ia) and attached algae make up the bulk of the standing crop of producers in this spring in which numerous aquatic insects, snails, herbivorous fish and turtles comprise the primary con- sumers. Other .fish and invertebrates form the smaller "crop" of sec- ondary consumers and bass and gar are the chief "top carnivores."
Animal parasites were included in the latter level. Since the de·
composers are primarily concerned with the breaking down of the large bulk of plants but also decompose all other levels as well, it is logical to show this component as a taU bar resting on the primary trophic level but extending to the top of the pyramid as well. Actually the biomass of decomposers is very small in relation to their importance in the functioning of the community. Thus, the pyramid of numbers greatly overrates the decomposers and the pyramid of biomass greatly underrates them. Neither numbers nor weights, in themselves, have much meaning in determining the role of decomposers in community dynamics; only measmements of actual energy utilization as could be shown on the energy pyra- mid will place the decomposers in true relationship with the macroscopic components. For this reaSOD, and also because few
ENERGY IN ECOLOGICAL SYSTEMS: §4 63 estimates of total microbial populations have actually been made, the decomposer unit is not included on the other biomass pyra- mids illustrated in Figure 15.
Comparison of standing crop pyramids of springs, lakes, coral reefs, open oceans and fields as shown in Figure 15 brings out a number of interesting pOints. The examples have been chosen to illustrate the range of conditions found in natme. For reasons already discussed, biomass pyramids are usually upright but may be partly inverted. Only when the bulk of producer organisms are very small, for example, tiny algae or photosynthetic bacteria, is an inverted pyramid to be expected. Such inverted pyramids have been mainly reported from open watcr communities where plants growing on the bottom were J)ot considered or where the water is
p. 80S SILVER SPRINGS, FLORIDA
L....-. _ _ _ _ _ _ _ ---'lp·703 P-470
' - - - ' CORAL REE~ ENIWETOK ATOLL OLD FIELD, GEORGIA
~
_-II paS6 4UNFERTILIZED
23 22 P -170 FERTILIZED WEBER LAKE, WISCONSIN
3 2
~
ZOOPLAN KTON ~ 2 I& BOTTOM F AU NA 16 PHYTOPLANKTON 4
LONG I SLAN D SOU NO ENGLISH CHANNEL GMSno1Z
Figure 15. Biomass pyramids of diverse aquatic and terrestrial ecosystems.
Pyramids are drawn apprOximately to same scale, and figures represent grams of dry biomass per square meter. P = producers, H
==
herbivores, C==
carnivores, TC
==
top carnivores, and D = decomposers. Pyramids cOn- structed from data as follows: Silver Sprjng.~-H. T. Odum, 1957; coral reef -Odum and Odum, 1955; old field-E. P. Odum, 1957a; Weber Lake-Juday, 1942; Long Island Sound-Riley, 1956; English Channel-Harvey, 1950.64 BASIC ECOLOGICAL PRINCIPLES AND CONCEPTS: CR. 3 too deep for bottom plants to exist (as in vast areas of the ocean).
In such situations of rapid turnover, the standing crop structure (of the lower trophic levels, at least) is likely to be highly vari- able from time to time, with biomass pyramids sometimes being inverted, sometimes upright. What the average situation is in many of these open water communities has not been determined.
In a recent study in which only the first two trophiC levels of the open water were considered, Pennak (1955) found that the ratio by weight of the consumcr crustaceans (zooplankton) to producer algae (phytoplankton) in 15 Colorado lakes varied from 0.4/1 to 9.9/1; in most lakes the standing crop of zooplankton was greater than the standing crop of phytoplankton on which they were feeding. In another study, Fleming and Laevastu (1956) report that ratio of zooplankton to phytoplankton in higher latitudes varies from III in winter to 1/2,5 in early summer, suggesting that the shape of the biomass pyramid may be expected to change with season.
If, for the moment, we may assume that the examples in Figure 15 are representative of the range of situations to be expected, \ve may make the following generalizations: (1) In tenestrial and shallow water ecosystems, where producers are large and rela- tively long-lived, a broad-based, relatively stable pyramid is to be expected. There is some evidence to show that pioneer or newly established communitics will tend to have fewer consumers in proportion to producers (i.e., the apex of the h.iomass pyramid will be small) as illustrated by the "old-field" pyramid in com- parison with that of the coral reef in Figure 15. Generally speak- ing, consumer animals in terrestrial and shallow water communi- ties have more complicated life histories and habitat requirements (specialized shelter, etc.) than do green plants; hence animal populations may require a longer period of time for maximum development. (2) In open water or deep water situations where producers are small and short-lived the standing crop situation at anyone moment may be widely variable and the biomass pyramid may be inverted. Also, the overall size of the total standing crop will likely be smaller (as indicated graphically by the area of the biomass pyramid) than that of land or shallow water communities, even if the total energy fixed annually is the same. Finally, (3) lakes and ponds where both large rooted plants and tiny algae are important may be expected to have an intermediate arrangement of standing crop units (as illustrated by Weber Lake, Fig. 15).
We may now turn to the consideration of energy pyramids as